Reduced size wireless communication systems designed for wearable or body worn applications are gaining acceptance for a wide variety of applications. Medical devices which monitor a patient's health and transmit in real-time diagnostic information is one example. Another example is a tracking system comprised of a GPS receiver and a cellular transceiver packaged in a small form factor to allow for convenient and non-intrusive attachment to a person, animal, or small asset. The tracking system uses GPS to determine location which is then sent out over the cellular network to provide real-time or near real-time asset tracking. Advances in transceiver and baseband design along with reduced size RF communication circuits allow for a small communication system to be assembled on a printed circuit board (PCB) and packaged in a plastic housing to provide a small form factor, robust communication device for body worn applications. For thee cellular, WiFi, and GPS transceiver and receiver circuits a small antenna is required to transmit and receive the RF signal.
The small PCB dimensions required for a small communication system for body worn devices can cause issues in regards to antenna design, placement, and performance. Internal antennas used for wireless applications are affected by the dimensions of the ground plane that the antenna operates off of. The circuit board of the wireless device is typically used as the ground plane for the antenna. Typically the circuit board dimensions are chosen to accommodate the wireless circuit and cannot be optimized for antenna performance. For common antenna types that are used for internal antennas in wireless mobile communications such as monopoles, IFAs, PIFAs, and isolated magnetic dipoles (IMD) a ground plane that is one quarter wavelength in extent is optimal for antenna performance. Odd multiples of one-quarter wavelength will also provide for optimal impedance and efficiency characteristics for these internal antenna types. As the linear dimensions of a ground plane, the length and width, decrease below a quarter wavelength the bandwidth and the efficiency of an antenna coupled to this ground plane will decrease. When the length or width of the ground plane drops below one-tenth of a wavelength the ground plane can be designated as being electrically small. If an antenna type requires a ground plane for operation, then the bandwidth and efficiency of an antenna coupled to an electrically small ground plane will be reduced due to an inadequate electrical extent presented by the small ground plane.
When a multi-frequency band antenna is designed for use on a small ground plane, the lowest frequency band of interest will be the most impacted by the small area and small length and width provided by the ground plane. As the upper frequency bands are considered improved antenna efficiency and bandwidth will be realized compared to the lowest frequency of operation. An antenna coupled to a small ground plane has the potential of radiating a substantial amount of power behind the ground plane, due to the reduced electrical size of the ground plane. If the antenna and ground plane are positioned in close proximity to a user, as in the case of a body worn wireless communication device, a substantial portion of the radiated power can couple to the user's body. This can result in reduced communication link efficiency between the body worn communication system and the other end of the communication link such as a cellular base station or WLAN access point. This can also result in increased power absorbed by the user, which in turn results in increased Specific Absorption Rate (SAR) levels coupled.